sys_stat: Support for MSVC. * lib/sys_stat.in.h (S_IFIFO): Define to _S_IFIFO if that exists. * tests/test-sys_stat.c: Don't assume that S_IFBLK exists. * doc/posix-headers/sys_stat.texi: Mention missing S_IFIFO, S_IFBLK on MSVC.